Shrub density effects on the presence of an endangered lizard of the Carrizo Plain National Monument, California

Abstract Positive associations between animals and foundational shrub species are frequent in desert ecosystems for shelter, resources, refuge, and other key ecological processes. Herein, we tested the impact of the density of the shrub species Ephedra californica on the presence and habitat use of the federally endangered lizard species, Gambelia sila. To do this, we used a 3‐year radio telemetry dataset and satellite‐based counts of shrub density across sites at the Carrizo Plain National Monument in San Luis Obispo County, CA. The effect of shrub density on lizard presence was contrasted with previous shrub cover analyses to determine whether measures of shrub density were superior to shrub cover in predicting lizard presence. Increasing shrub density increased lizard presence. As shrub density increased, lizards were located more frequently “above ground” versus “below ground” in burrows. Male lizards had significantly larger home ranges than females, but both sexes were similarly associated with increasing shrub densities. Shrub density and shrub cover models did not significantly differ in their prediction of lizard presence. These findings suggest that both habitat measures are effective analogs and that ecologically, both cover and the density of foundation shrub species are key factors for some desert lizards.

and biotic stressors primarily drive the dependency of organisms on ecosystem resources (Milchunas & Noy-Meir, 2002;Nelson et al., 2007). Physiologically, reptilian species are highly dependent on thermoregulatory processes using exterior resources as they cannot maintain their internal temperature through metabolic heat (Ivey et al., 2020;Sunday et al., 2014). Thermoregulating reptile species such as lizards and snakes overtly exhibit this dependency, as extreme temperatures can only be avoided by seeking refuge above ground in/under vegetation or below ground in burrows, thus making these excellent species to study how resources are used within an ecosystem (Dematteis et al., 2022;Germano, 2019;Ivey et al., 2020;Sunday et al., 2014;Urban, 2015).
The home range for G. sila varies depending on the sex of the lizard but can range between 4 and 6 ha for males and 1 and 8 ha for females (Germano & Rathbun, 2016;Westphal et al., 2018). These home ranges are typically influenced by the presence and number of shrubs, suggesting that there may be an optimal number of shrub individuals that is sufficient for G. sila habitats .
With a higher number of shrubs within these areas, there is more opportunity for this species to thermoregulate as well as find food and shelter from predation Westphal et al., 2018). However, previous work by Germano and Rathbun (2016) suggests that while the home ranges of G. sila contain more shrub habitat, it is still possible for this species to travel and establish home ranges in the absence of shrubs. Challenges to thermoregulation-and dependency on the local environment for refuge-for lizards, including G. sila, are becoming more acute as global temperatures and the likelihood of drought events increase (Dell et al., 2014;Lortie et al., 2020;Sinervo et al., 2010;Westphal et al., 2016). Lortie et al. (2020) found that there was a significant association between increasing shrub cover, the Normalized Difference Vegetation Index (NDVI), and G. sila presence-that is, an individual observation of each individual during data collection. To build off that work and to further examine the relationship between shrub density and lizard presence (Germano et al., 2011), we examined here whether shrub density could also be used, in addition to cover, to predict G. sila presence. Shrub density was selected for this study to further build on the finding previously reported by Zuliani et al. (2021Zuliani et al. ( , 2023 that shrub density influences the abundance and richness of local desert species while acting as an indicator of overall animal abundance. Here, we hope to extend these findings to a federally endangered species and show that shrub density is easier-and thus potentially cheaper and more cost effective for rapid rangewide surveys-to measure, both in the field and with satellite-based data. While studies have evaluated the importance of shrub density on other vertebrate species within arid ecosystems (Zuliani et al., , 2023, previous studies evaluating the importance of shrub density on G. sila presence relied on observational data  and thus were not broadly applicable. To address these deficiencies and to increase the potential for an approach that could be more easily scaled across G. sila's range, we developed methodology that uses radio telemetry and other G. sila presence data previously collected by Westphal et al. (2016) and Lortie et al. (2020) together with novel satellite-based shrub density measurements to test the potential of free, easily accessible satellite data for predicting G. sila presence across large suitable landscapes. In this paper, we specifically examined the following questions: 1. Does G. sila presence increase with shrub density? 2. Do shrub density and cover similarly predict G. sila presence?
3. Is G. sila more likely to be observed above ground than below ground as shrub density increases? 4. Do home range and mean annual travel distance differ between male and female G. sila? 2 | ME THODS

| Study species
Ephedra californica is the dominant shrub species within the Elkhorn Plain in the Carrizo Plain National Monument in San Luis Obispo County, CA, USA (35.11982, −119.62853) . Ephedra californica can reach heights of up to 1 m and typically takes 5-10 years to reach 0.5 m in size, suggesting that its cover and density changes over a longer period (Cutler, 1939;Filazzola et al., 2020;. With numerous twigs and needle-like leaves, this species possesses unique characteristics of both gymnosperms and angiosperms, making it well-adapted to its native semi-arid and arid environments (Loera et al., 2012). Ephedra californica is considered a foundational shrub species because it plays a disproportionately large role in structuring the system (Hawbecker, 1951;Westphal et al., 2018;Zuliani et al., 2021). This shrub species provides several unique resources to local lizard populations, including as a refuge from predators and as a place for thermoregulation Gaudenti et al., 2021;Ivey et al., 2020;Lortie et al., 2018;Zuliani et al., 2021).
Within the Carrizo Plain National Monument, G. sila are found in both shrubbed and open areas in isolated populations (Gaudenti et al., 2021;Lortie et al., 2020;Westphal et al., 2018). Male G. sila individuals are territorial, minimally overlapping with other males (Germano, 2019;Westphal et al., 2018). Gambelia sila are mainly insectivores; however, they may consume smaller lizard species (Germano, 2019;Warrick et al., 1998;Westphal et al., 2018). The active season for G. sila adults lasts only ~3 months in the late spring to early summer, after which they transition into hibernation (Germano et al., 1994;Ivey et al., 2020). Gambelia sila is associated with shaded areas under shrub species and in underground burrows , which they use as an additional way to thermoregulate (Gaudenti et al., 2021;Germano, 2019;Ivey et al., 2022). Gambelia sila will spend nights within burrows and may return to these burrows during the day when temperatures become too high (Warrick et al., 1998;Westphal et al., 2018).

| Radio telemetry
We used G. sila location data previously collected using radio telemetry at the Carrizo Plain National Monument from 2016 to 2018 (Lortie et al., 2019Noble et al., 2016;Westphal et al., 2018;Zuliani et al., 2021). Gambelia sila location data were collected on a total of 62 individuals, 36 males and 26 females. Gambelia sila individuals were tracked for 3 months (May-July) each year (2016)(2017)(2018) during the time period of greatest activity for this species Westphal et al., 2018). Holohil model BD-2 tags were attached to G. sila individuals using a small beaded chain, jewelry wire, and epoxy . Collar weight (with the tag) ranged from 1.6 to 2.2 g, ensuring that the weight did not exceed between 5% and 10% of the lizard's body mass .
Each instance of G. sila presence was geolocated. Individual characteristics such as an individual's sex were also collected during the initial capture. Individuals located above ground and active were designated as "above ground," while individuals judged to be below ground were designated as "below ground." At each location, the presence of shrubs was recorded. Gambelia sila association with a shrub individual or in open areas was then recorded as microsite.
Within the 3 years of observations, a total of 3553 G. sila observations were recorded with 1502 above ground and 2051 below ground Westphal et al., 2016). Ephedra californica is a long-lived, slow-growing shrub species and will thus not change in size and density over 3 years of continuous sampling (Bowers et al., 1995), allowing us to compare lizard presence and shrub density data from slightly different time periods. Additional details on the telemetry procedures can be found in Westphal et al. (2018). The transportation, care, and use of lizards were in accordance with the Animal Welfare Act (7 U.S.C. 2131 et. seq.), which guides the US government use of vertebrates. We ameliorated any suffering of captured animals by allowing only trained personnel to handle them, and by limiting handling and housing time to the minimum necessary.

| Shrub density data collection and ground truthing
Ephedra californica density data were derived from Google Earth using composite satellite imagery-digital images comprised of elements from several different images-from 2021. Composite imagery was developed using Landsat/Copernicus satellite data, with a 30 m spatial resolution. Satellite imagery was obtained for the same loca-  (Zaitunah et al., 2018) and was included here as it is known to be a strong predictor of vegetation greenness and primary production (Butterfield & Malmström., 2009;Ju & Masek, 2016) and is therefore commonly used to quickly identify vegetated areas (Zaitunah et al., 2018). NDVI and shrub density were then both used to determine whether these factors influenced G. sila presence. All data are publicly published and available on the Knowledge Network of Biodiversity .

| S TATIS TIC AL ANALYS IS
All statistics and models were done in R 4.2.1 (R Core Team, 2022).
The ResourceSelection R package (Lele et al., 2019) was used to model G. sila shrub use based on density and cover . The distinct function was used in base R 4.2.1 to remove any duplicates within the lizard presence data. Data were filtered and compared based on similarity of latitude, longitude, lizard identification, year, and microsite. Individuals with identical latitude and longitude coordinates within the same year were removed to minimize the probability of duplication. In addition, individuals who were located at the same latitude and longitude associating with the same microsite were also filtered. We also tested the effects of rounding the latitude and longitude to 4 and 3 decimal points on lizard duplicate removal. Rounding the lizard geolocations to 4 decimal places reduces the distance of each coordinate by 11 m while rounding the geolocations to 3 decimal places reduces the distance of each coordinate by 111 m. This was conducted in a variety of combinations and compared with the raw data. The SF package was used to calculate the mean annual distance traveled by individual lizards using three different duplication removal methods (Table S1; Pebesma, 2018).
These methods were selected as they provided the most meaningful data after filtering for possible duplicates. 1000 pseudo-absence data points were then generated using the dismo R package to simulate areas where G. sila individuals were not located (Hijmans et al., 2022). The resource selection probability function (rspf) was used to estimate the probability of a lizard individual in shrubbed areas (Lele et al., 2013;Roberts et al., 2017). In addition, sex and year were used as a factor with shrub density to estimate the resource selection of shrub density between male and female individuals across 3 years of telemetry data. This function estimates the frequency of occurrence of a species for a specific factor such as shrub density and cover (Roberts et al., 2017). Akaike information criterion (AIC) scores were generated for both shrub density and shrub cover models and compared as both strongly correlated with lizard shrub use (Lele et al., 2013;Lortie et al., 2020;Roberts et al., 2017). The scores determined which model best suited the data-based on a lower AIC score. Pearson's Product-Moment Correlation was used to determine the strength and directions of the relationship between shrub density and cover. An approximation of second derivative for the data points for the mean fitted use by shrub density curve was used to determine the inflection point of fit (Christopoulos, 2014).
Maximum likelihood estimates were used with shrub density, shrub cover, and ground use (above vs. below) as predictor variables. Shrub density and cover were analyzed against lizard presence to depict their relationship. Home range sizes were then calculated using 95% and 100% minimum convex polygon areas, using the adehabitatHR package (Calenge, 2006;Mohr, 1947). Both male and female G. sila home ranges were calculated and compared across the 3 years of telemetry data. General linear models were then used to compare shrub density and shrub cover use by both male and female G. sila individuals.

| RE SULTS
There were no statistically significant differences between the ge-

F I G U R E 1
Relationship between shrub density (number of Ephedra californica individuals per 20 m 2 radius) and shrub cover for E. californica within the Carrizo Plain National Monument study site. Total shrub density was joined with estimate shrub cover from imagery data to test for a correlation (r = .54, p-value < .001).
t-value = 23.761, df = 3571, p < .001). The AIC score for the shrub density rsf model was lower than the shrub cover model, suggesting that density is a more parsimonious model fit for presence data (Snipes & Taylor, 2014) (Table S2; AIC scores density = 11,253.37, cover = 11,257.06). Shrub density significantly predicted the presence of G. sila, for both the above ground and below ground categories    Table S4; t-test = −2.318. df = 56, p = .0241).

| DISCUSS ION
The major finding of this study is that the presence of G. sila individuals increased with higher shrub density, up to a plateau, which we identified was 100 shrubs per 20 m 2 , or 25% shrub cover . Higher shrub densities were also associated with increased likelihood of lizard presence "above ground" versus "below ground." Measurements of NDVI in our shrub density models positively predicted the presence of G. sila individuals, similar to conclusions drawn from previous work by Lortie et al. (2020) showed that both shrub cover and NDVI positively predicted lizard presence.
Male lizards had larger home ranges than female individuals for the 2018 sampling season only. Males were frequently associated with higher density shrub areas, but the predicted habitat use was not significantly different between lizard sexes, suggesting that shrub density and cover are important as potential resources for both male and female G. sila Ivey et al., 2022). Increasing shrub densities increases the likelihood that a lizard will encounter and interact with a shrub , providing stopping points or distributed refuges for individuals as they move within their home range.
Landscape features and various natural resources are important factors to consider in ecological surveys that observe associations.
Typically, in arid/semi-arid ecosystems, shrub species provide benefits to local animal communities through buffering climatic extremes, acting as refuges from potential predation, and other abiotic and biotic conditions (Bruno et al., 2003;Eldridge & Soliveres, 2014;Filazzola et al., 2017;Ivey et al., 2022;Noble et al., 2016;Westphal et al., 2018). Areas with higher shrub cover allow for lower thermal amplitudes, while areas with higher density can consist of smaller shrubs utilized for abiotic stress amelioration, which provides multiple areas for individual utilization (Hollzapfel & Mahall, 1999;Filazzola et al., 2017). This behavior extends beyond G. sila as several other reptile species utilize shrub cover and density to ameliorate abiotic stressors. For instance, Psammodromus algirus, the F I G U R E 2 Relative effects of shrub density (number of Ephedra californica individuals per 20 m 2 radius) on Gambelia sila presence probability above and "below ground" for each individual presence. Radio telemetry data were joined with geolocated shrub data to estimate the shrub density within a 20 m radius of each Gambelia observation. The data were then grouped by above and "below ground." Shaded areas show a 95% confidence interval band for the lines of best fit. Algerian sand racer, utilizes shrubs within the Tajo Basin of Spain (Diaz & Carrascal, 1991;Zamora-Camacho et al., 2016) for thermoregulation, predator avoidance, and movement minimization (Diaz & Carrascal, 1991). These facilitative associations, in which one species benefits while the other is unaffected (Molina-Montenegro et al., 2016), can be subdivided into facultative (being able to live with the presence or absence of an environmental condition), and obligate, where one species requires the benefits from another to exist (Butterfield, 2009). The relationship observed in our study is more facultative than obligate, as G. sila will utilize shrubs when present; however, if absent, individuals can still be present within a site.
With more shrubs, there are likely more opportunities for positive associations as even relatively small shrub individuals provide benefits (Gaudenti et al., 2021;Lortie et al., 2018;Zuliani et al., 2021).
Shrubs can provide refuge for lizards to thermoregulate and escape harsh abiotic conditions, so that they might stay above ground even when below ground temperatures are more optimal (Gaudenti et al., 2021). Shrub cover and density therefore potentially provide crucial opportunities for lizard thermoregulation and can be utilized to predict the frequency of individual lizard observations (Filazzola et al., 2017;Lortie et al., 2020;Zuliani et al., 2021). This contrasts with lizard individuals in open or relatively shrub-free habitats, which are more reliant on burrows to reduce their body temperature, similar to other desert species (Bean et al., 2014;Gaudenti et al., 2021;Germano & Rathbun, 2016;Ivey et al., 2020). Males also generally extended their home ranges in this ecosystem, suggesting that refuge provided by foundational plant species can be important stopping or foraging points (Ivey et al., 2020(Ivey et al., , 2022. While female G. sila individuals similarly utilize these areas for foraging points, a large portion of their active season is spent underground while breeding (Germano, 2019;Ivey et al., 2020). The influence of shrub density on the thermoregulation, behavior, and associations of blunt-nosed leopard lizards with habitat further illustrates how these resources are beneficial to this endangered species and how important relevant associations are to their conservation and restoration.  (Bryant et al., 2020;Butterfield et al., 2021;Kelsey et al., 2018). Restoring these ecosystems could lead to G. sila-as well as more than 20 other threatened and endangered species-recovery. Our findings can help guide these restoration efforts, identifying specific density and cover thresholds that will provide the best potential habitat for G. sila individuals.

F I G U R E 3
Relative home range of Gambelia sila individuals (ha 2 ) across 3 years of radio telemetry tracking. Home range sizes were calculated using the adehabitatHR package in R 4.2.1. Individuals were separated based on lizard sex with blue representing females and red representing males. Dashed lines represent the average home range of male and female individuals independently.